BDChapter 3: Chemical Reactions 69

VOCABULARY

chemical reaction p. 69

reactant p. 71

product p. 71

precipitate p. 72

catalyst p. 76

BEFORE, you learned

• Atoms of one element differfrom atoms of all other elements

• Chemical bonds hold compounds together

• Chemical bonds may be ionicor covalent

NOW, you will learn

• About chemical changes andhow they occur

• About three types of chemicalreactions

• How the rate of a chemicalreaction can be changed

KEY CONCEPT

Chemical reactions alterarrangements of atoms.

Atoms interact in chemical reactions.

You see substances change every day. Some changes are physical, such

as when liquid water changes to water vapor during boiling. Other

changes are chemical, such as when wood burns to form smoke and

ash, or when rust forms on iron. During a chemical change, substances

change into one or more different substances.

A produces new substances by changing the

way in which atoms are arranged. In a chemical reaction, bonds

between atoms are broken and new bonds form between different

atoms. This breaking and forming of bonds takes place when particles

of the original materials collide with one another. After a chemical

reaction, the new arrangements of atoms form different substances.

chemical reaction

COMBINATION NOTES

Use combination notes to organize informationabout how atoms interact during chemical reactions.

EXPLORE Chemical Changes

How can you identify a chemical change?

PROCEDURE

Pour about 3 cm (1 in.) of vinegar into the

bowl. Add a spoonful of salt. Stir until the

salt dissolves.

Put the pennies into the bowl. Wait two

minutes, and then put the nail into the bowl.

Observe the nail after five minutes and

record your observations.

WHAT DO YOU THINK?• What did you see on the nail? Where do you

think it came from?

• Did a new substance form? What

evidence supports your conclusion?

3

2

1

MATERIALS• vinegar

• clear bowl

• plastic spoon

• table salt

• 20 pennies

• large iron nail

Page 1 of 8

BD 70 Unit: Chemical Interactions

Physical Changes

A change in the state of a substance is an example of a physical change.

The substance may have some different properties after a physical

change, but it is still the same substance. For example, you know that

water can exist in three different physical states: the solid state (ice), the

liquid state (water), and the gas state (water vapor). However, regardless

of what state water is in, it still remains water, that is, H2O molecules.

As ice melts, the molecules of water move around more quickly, but

the molecules do not change. As water vapor condenses, the molecules

of water move more slowly, but they are still the same molecules.

Substances can undergo different kinds of physical changes.

For example, sugar dissolves in water but still tastes sweet because

the molecules that make up sugar do not change when it dissolves.

The pressure of helium changes when it is pumped from a high-

pressure tank into a balloon, but the gas still remains helium.

Check Your Reading What happens to a substance when it undergoes a physical change?

Liquid water is composedof molecules that move freelypast each other.

Ice is composed of watermolecules that are lockedtogether.

When water changes from a liquid to a solid, it undergoesa physical change.

Page 2 of 8

BDChapter 3: Chemical Reactions 71

Chemical Changes

Water can also undergo a chemical change. Water

molecules can be broken down into hydrogen and

oxygen molecules by a chemical reaction called

electrolysis. When an electric current is passed through

liquid water (H2O), it changes the water into two

gases—hydrogen and oxygen. The molecules of

water break apart into individual atoms, which then

recombine into hydrogen molecules (H2) and oxygen

molecules (O2). The original material (water) changes

into different substances through a chemical reaction.

Hydrogen and oxygen are used as rocket fuel for the

space shuttle. During liftoff, liquid hydrogen and liquid

oxygen are combined in a reaction that is the opposite

of electrolysis. This reaction produces water and a large

amount of energy that helps push the shuttle into orbit.

Check Your Reading How does a chemical change differ from a physical change?

Reactants and Products

are the substances present at the beginning of a chemical

reaction. In the burning of natural gas, for example, methane (CH4) and

oxygen (O2) are the reactants in the chemical reaction. are the

substances formed by a chemical reaction. In the burning of natural gas,

carbon dioxide (CO2) and water (H2O) are the products formed by

the reaction. Reactants and products can be elements or compounds,

depending on the reaction taking place.

During a chemical reaction, bonds between atoms in the reactants

are broken and new bonds are formed in the products. When natural

gas is burned, bonds between the carbon and hydrogen atoms in meth-

ane are broken, as are the bonds between the oxygen atoms in oxygen

molecules. New bonds are formed between carbon and oxygen in

carbon dioxide gas and between hydrogen and oxygen in water vapor.

Reactants—bonds broken Products—new bonds formed

methane + oxygen carbon dioxide + water

(CH4) (O2) (CO2) (H2O)

Check Your Reading What must happen for reactants to be changed into products?

Products

Reactants

Electrolysis of Water

hydrogen gas (H2)

water (H2O)

oxygen gas (O2)

Water molecules can be split apart to formseparate hydrogen and oxygen molecules.

O O

O C O

O O

OH H

OH H

H

C

H

HH

Page 3 of 8

BD 72 Unit: Chemical Interactions

Evidence of Chemical Reactions

Some chemical changes are easy to observe—the products formed

by the rearrangement of atoms look different than the reactants.

Other changes are not easy to see but can be detected in other ways.

Color Change Substances often change color during a chemical

reaction. For example, when gray iron rusts the product that forms

is brown, as shown in the photograph below.

Formation of a Precipitate Many chemical reactions form products

that exist in a different physical state from the reactants. A solid product

called a may form when chemicals in two liquids react, as

shown in the photograph below. Seashells are often formed this way

when a sea creature releases a liquid that reacts with seawater.

Formation of a Gas Chemical reactions may produce a gas, like that

often formed when antacid pills are mixed with excess stomach acid.

The photograph below shows an example in which carbon dioxide gas

is produced by a chemical reaction.

Temperature Change Most chemical reactions involve a temperature

change. Sometimes this change can be inferred from the observation

of a flame, as in the burning of the metal magnesium in the photograph

below. Other temperature changes are not immediately obvious. If you

have touched concrete before it hardens, you may have noticed that it

felt warm. This warmth is due to a chemical reaction.

precipitate

Color Change Formation of a Precipitate

Temperature ChangeFormation of a Gas

VOCABULARY

Remember to use a four square diagram forprecipitate and othervocabulary terms.

Page 4 of 8

BDChapter 3: Chemical Reactions 73

Chemical reactions can be classified.

Scientists classify chemical reactions in several ways to help make the

different types of reactions easier to understand. All reactions form

new products, but the ways in which products are made can differ.

Synthesis In a synthesis reaction, a new compound is formed by the

combination of simpler reactants. For example, nitrogen dioxide (NO2),

a component of smog, forms when nitrogen and oxygen combine in

the air.

Decomposition In a decomposition reaction, a reactant breaks down

into simpler products, which could be elements or other compounds.

Decomposition reactions can be thought of as being the reverse of

synthesis reactions. For example, water can be decomposed into its

elements—hydrogen and oxygen.

Combustion In a combustion reaction, one reactant is always oxygen

and another reactant often contains carbon and hydrogen. The carbon

and hydrogen atoms combine with oxygen, producing carbon dioxide

and water. The burning of methane is a combustion reaction.

Check Your Reading How are synthesis reactions different from decomposition reactions?

reading tip

Synthesis means “making a substance from simplersubstances.”

reading tip

Decomposition means “separation into parts.”

reading tip

Combustion is the processof burning with oxygen.

N2 + 2O2 2NO2

2H2O 2H2 + O2

N NN

N

O O

O O

O O

O O

O O

O O

O C O

O O

OH H

OH H

OH H

H H

H H

CH4 + 2O2 CO2 + 2H2O

H

C

H

H H

HO

H

Page 5 of 8

BD 74 Unit: Chemical Interactions

The rates of chemical reactions can vary.

Most chemical reactions take place when particles of reactants collide

with enough force to react. Chemical reactions can occur at different

rates. Striking a match causes a very quick chemical reaction, while

the rusting of an iron nail may take months. However, the rate of a

reaction can be changed. For instance, a nail can be made to rust more

quickly. Three physical factors—concentration, surface area, and tem-

perature—and a chemical factor—a catalyst—can greatly affect the

rate of a chemical reaction.

Concentration

Concentration measures the number of particles present in a certain

volume. A high concentration of reactants means that there is a large

number of particles that can collide and react. Turning the valve on a

gas stove to increase the flow of gas increases the concentration of

methane molecules that can combine with oxygen in the air. The

result is a bigger flame and a faster combustion reaction.

Surface Area

Suppose one of the reactants in a chemical reaction is present as a

single large piece of material. Particles of the second reactant cannot

get inside the large piece, so they can react only with particles on the

surface. To make the reaction go faster, the large piece of material

could be broken into smaller pieces before the reaction starts.

Chemical ReactionsChemical Reactions

SKILL FOCUSInferring

MATERIALS• 2 seltzer tablets• 2 plastic cups• tap water• stopwatch

TIME15 minutes

VISUALIZATIONCLASSZONE.COM

Observe how changingthe concentration of areactant can change therate of a reaction.

How can the rate of a reaction be changed?

PROCEDURE

Place a whole seltzer tablet in one cup. Crush the second tablet and

place it in the second cup.

Fill each cup halfway with water.

Time how long the tablet in each cup fizzes.

WHAT DO YOU THINK?

• How long did the whole tablet fizz? What about the crushed tablet?

• How are these results related to the rate of a chemical reaction? Explain.

CHALLENGE How might your results be related to

collisions between particles during a chemical reaction?

3

2

1

Page 6 of 8

BD

Breaking a large piece of material into smaller parts increases the

surface area of the material. All of the inner material has no surface

when it is inside a larger piece. Each time the large piece is broken,

however, more surfaces are exposed. The amount of material does not

change, but breaking it into smaller parts increases its surface area.

Increasing the surface area increases the rate of the reaction.

Check Your Reading Why does a reaction proceed faster when the reactants havegreater surface areas?

Temperature

The rate of a reaction can be increased by making the particles move

faster. The result is that more collisions take place per second and

occur with greater force. The most common way to make the particles

move faster is to add energy to the reactants, which will raise their

temperature.

Many chemical reactions during cooking go very slowly, or do not

take place at all, unless energy is added to the reactants. Too much heat

can make a reaction go too fast, and food ends up burned. Chemical

reactions can also be slowed or stopped by decreasing the temperature

of the reactants. Again, think about cooking. The reactions that take

place during cooking can be stopped by removing the food from the

heat source.

Concentration An increase inconcentration of the reactantsincreases the number of particles that can interact.

Changes in Reactants

Normal Reaction Rate

Increased Reaction Rate

Surface area An increase inthe surface area of the reac-tants increases the number of particles that can interact.

Temperature Adding energymakes particles move fasterand increases temperature.The increase in motion allowsreactants to collide and reactmore frequently.

Particles and Reaction Rates

reminder

Temperature is the averageamount of kinetic energy ofthe particles in a substance.

Chapter 3: Chemical Reactions 75

Page 7 of 8

BD 76 Unit: Chemical Interactions

Catalysts

The rate of a reaction can be changed chemically by adding a catalyst.

A is a substance that increases the rate of a chemical reaction

but is not itself consumed in the reaction. This means that after the

reaction is complete, the catalyst remains unchanged. Catalysts are

very important for many industrial and biological reactions. In fact,

many chemical reactions would proceed slowly or not take place at all

without catalysts.

In living things, catalysts called enzymes are absolutely necessary

for life. Without them, many important reactions could not take place

under the conditions within your body. In fact, in 2003, scientists

reported that they had discovered the slowest known chemical reaction

in living things. This reaction would normally take one trillion years.

Enzymes, though, allow the reaction to occur in 0.01 seconds.

Check Your Reading Why are catalysts important in chemical reactions?

catalyst

KEY CONCEPTS

1. How do physical changes differ

from chemical changes?

Explain.

2. Describe four types of evidence

of a chemical reaction.

3. Describe the ways in which the

rate of a chemical reaction can

be changed.

CRITICAL THINKING

4. Synthesize What evidence

shows that the burning of

methane is a chemical reaction?

5. Compare What about

combustion reactions makes

them different from either

synthesis or decomposition

reactions?

CHALLENGE

6. Apply How might the chewing

of food be related to the rate

of a chemical reaction—

digestion—that occurs in your

body? Explain.

An enzyme is a catalystfor chemical reactions inliving things.

Enzymes allow reactionsthat would not normallytake place to occur.

A new product is made,but the enzyme is notchanged by the reaction.

enzyme(catalyst)

reactants

reactantscombined

unchangedenzyme

new product

1 2 3

RESOURCE CENTER

CLASSZONE.COM

Learn more about cata-lysts and how they workin living things.

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